On Thu, Nov 18, 2010 at 09:58:31AM -0800, Andrew Morton wrote: > On Thu, 18 Nov 2010 19:18:22 +1100 Nick Piggin <npiggin@xxxxxxxxx> wrote: > > > On Wed, Nov 17, 2010 at 10:28:34PM -0800, Andrew Morton wrote: > > > > > Logically I'd expect i_mutex to nest inside s_umount. Because s_umount > > > is a per-superblock thing, and i_mutex is a per-file thing, and files > > > live under superblocks. Nesting s_umount outside i_mutex creates > > > complex deadlock graphs between the various i_mutexes, I think. > > > > You mean i_mutex outside s_umount? > > > > Nope. i_mutex should nest inside s_umount. Just as inodes nest inside > superblocks! Seems logical to me ;) Right, but your last sentence seemed to suggest that the natural ordering creates deadlocks :) > > > And why _do_ we need to hold s_umount during the bdi_queue_work() > > > handover? Would simply bumping s_count suffice? > > > > s_count just prevents it from going away, but s_umount is still needed > > to keep umount, remount,ro, freezing etc activity away. I don't think > > there is an easy way to do it. > > > > Perhaps filesystem should have access to the dirty throttling path, kick > > writeback or delayed allocation etc as needed, and throttle against > > outstanding work that needs to be done, going through the normal > > writeback paths? > > I just cannot believe that we need s_mount inside ->write_begin. Is it > really the case that someone can come along and unmount or remount or > freeze our filesystem while some other process is down performing a > ->write_begin against one of its files? Kidding? Not for the work handoff either? If that is all waited on synchronously before ->write_end returns, then no we shouldn't need any more locks of course. But asynch writeout needs a mutex rather than refcount so the umount has something to block against and not just fail. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
